The present invention is soluble silicone resin compositions having good solution stability and a method for their preparation. The present silicone resins are useful to form microporous films having low dielectric constants.
Semiconductor devices often have one or more arrays of patterned interconnect levels that serve to electrically couple the individual circuit elements forming an integrated circuit (IC). These interconnect levels are typically separated by an insulating or dielectric film. Previously, a silicon oxide film formed using chemical vapor deposition (CVD) or plasma enhanced techniques (PECVD) was the most commonly used material for such dielectric films. However, as the size of circuit elements and the spaces between such elements decreases, the relatively high dielectric constant of such silicon oxide films is inadequate to provide adequate electrical insulation.
In order to provide a lower dielectric constant than that of silicon oxide, dielectric films formed from siloxane-based resins have found use. An example of such films are those formed from poly(hydrogen)silsesquioxane resins as described for example in Collins et al., U.S. Pat. No. 3,615,272, and Haluska et al. U.S. Pat. No. 4,756,977. While such films provide lower dielectric constants than CVD or PECVD silicon oxide films and also provide other benefits such as enhanced gap filling and surface planarization, typically the dielectric constants of such films are limited to approximately 3, or greater.
It is well known that the dielectric constant of the above discussed insulating films is an important factor where IC's with low power consumption, cross-talk, and signal delay are required. As IC dimensions continue to shrink, this factor increases in importance. As a result, siloxane based resin materials and methods for making such materials that can provide electrically insulating films with dielectric constants below 3, are desirable. In addition, it is desirable to have siloxane-based resins and method for making such resins that provide low dielectric constant films which have a high resistance to cracking. Also, it is desirable for such siloxane-based resins to provide low dielectric constant films by standard processing techniques.
It is known that the dielectric constant of solid films decrease with a decrease in density of the film material. Therefore considerable work is being conducted to develop microporous insulating films for use on semiconductor devices.
Kapoor, U.S. Pat. No. 5,494,859, describes a low dielectric constant insulating layer for an integrated circuit structure and a method of making the layer. A porous layer is formed by depositing on a structure a composite layer comprising an insulating matrix material and a material which can be converted to a gas upon subjection to a converting process. Release of the gas leaves behind a porous matrix of the insulating material which has a lower dielectric constant than the composite layer. The matrix forming material is typically silicon oxide and the material which can be converted to a gas upon subjection to a converting process is exemplified by carbon.
Hedrick et al., U.S. Pat. No. 5,776,990, describe an insulating foamed polymer having a pore size less than about 100, nm made from a copolymer comprising a matrix polymer and a thermally decomposable polymer by heating the copolymer above the decomposition temperature of the decomposable polymer. The copolymers described are organic polymers that do not contain silicon atoms.
Smith et al., WO 98/49721, describe a process for forming a nanoporous dielectric coating on a substrate. The process comprises the steps of blending an alkoxysilane with a solvent composition and optional water; depositing the mixture onto a substrate while evaporating at least a portion of the solvent; placing the substrate in a sealed chamber and evacuating the chamber to a pressure below atmospheric pressure; exposing the substrate to water vapor at a pressure below atmospheric pressure and then exposing the substrate to base vapor.
Mikoshiba et al., Japanese Laid-Open Patent (HEI) 10-287746, describe the preparation of porous films from siloxane-based resins having organic substituents which are oxidized at a temperature of 250.degree. C. or higher. The useful organic substituents which can be oxidized at a temperature of 250.degree. C. or higher given in this document include substituted and unsubstituted groups as exemplified by 3,3,3-trifluoropropyl, .beta.-phenethyl group, t-butyl group, 2-cyanoethyl group, benzyl group, and vinyl group.
Mikoskiba et al., J. Mat. Chem., 1999, 9, 591-598, report a method to fabricate angstrom size pores in poly(methylsilsesquioxane)films in order to decrease the density and the dielectric constant of the films. Copolymers bearing methyl(trisiloxysilyl) units and alkyl(trisiloxysilyl) units are spin-coated on to a substrate and heated at 250.degree. C. to provide rigid siloxane matrices. The films are then heated at 450.degree. C. to 500.degree. C. to remove thermally labile groups and holes are left corresponding to the size of the substituents. Trifluoropropyl, cyanoethyl, phenylethyl, and propyl groups were investigated as the thermally labile substituents.
Hacker et al., WO 98/47945, teach a method for reacting trichlorosilane and organotrichlorosilane to form organohydridosiloxane polymer having a cage conformation and between approximately 0.1, to 40, mole percent carbon-containing substituents. Resin formed from the polymers are reported to have a dielectric constant of less than about 3.
The objectives of the present invention include providing a silicone resin which is soluble in organic solvents such as toluene, has a useable solution shelf-life, and which is suitable for forming crack-free electrically insulating films on electronic devices. Another objective is to provide a silicone resin composition which after coating on a substrate can be heated to form a microporous film have a narrow pore size distribution and a low dielectric constant. Such low-dielectric constant films can be formed on electrical components such as semiconductor devices by conventional methods to form microporous crack-free films having a dielectric constant less than about 2.